The present invention is related generally to digital signal processing and, in particular, digital signal processing of images.
Many modern-day image processors utilize compression (or coding) techniques to minimize the amount of memory needed to store and process the images. The compression techniques used are those typically established by the International Standards Organization. For example, a Joint Photographic Experts Group (JPEG) of the International Standards Organization developed a JPEG digital data compression technique for editing still images, as well as for use in color facsimile (FAX) machines, and medical imaging systems. Digital image recorders use a compression technique developed by a Moving Pictures Expert Group (MPEG) to minimize the amount of data stored when acquiring or processing moving (or motion) pictures. An MPEG-1 standard generally is used for relatively low-level resolution, low-bit rate image sequences. An MPEG-2 standard has been developed to accommodate relatively high resolution, high-bit rate image sequences of the type generally used for full screen playback, such as in entertainment systems, web browsers, television (TV), high-definition TV (HDTV), or with image games played on personal computers (PCs).
These compression techniques typically divide an image into blocks of pixels (block coding) for processing. During processing, a well known discrete cosine transform (DCT) is applied to each block of pixels, the results of which are quantized and encoded. When the image is to be displayed, on a TV or PC, for example, it must be decoded in advance. Depending on the selected bit rate (i.e., compression factor), the images present noticeable degradation due to quantization errors. For example, when the compression ratio is increased, the artifacts of block coding the image become visible around the block boundaries. These blocking artifacts primarily consist of two types of artifacts: grid noise and staircase noise. When processing the image, the noise has characteristics similar to those of an image""s fine details, such as the image""s edges and textures. Any filtering of the noise thus usually filters the image""s edges and textures, as well, causing blurring. It can be appreciated therefore that what is needed is a technique to reduce the blocking artifacts without blurring the image""s fine details.
Presented herein is a system and method to reduce blocking artifacts that overcome the limitations associated with image compression standards. An example embodiment stores several rows of an image to be processed in line memories. The image rows contain a target pixel as well as pixels neighboring the target pixel. This exemplar extracts global metrics from the image, estimates local metrics related to the target pixel and the neighboring pixels as local metrics, uses fuzzy logic to determine the number of pixels to be processed, determines the difference in gray level values between the target pixel and the neighboring pixels, and smoothes any abrupt transitions in gray levels from the target pixel to neighboring pixels using a dual ramp generator.
Further features and advantages as well as the structure and operation of various embodiments are described in detail below.